The present invention relates to a double monochrometer in which a pair of monochrometers are series-connected for eliminating stray light.
One example of the conventional double monochrometers is shown in FIG. 1. Referring to FIG. 1, light entering a first slit 1 impinges on a spherical mirror 8 from which a substantially collimated light flux is directed to a plane grating 4. Light diffracted at the grating 4 is focused by a spherical mirror 9 and passes through a plane mirror 6 to a second slit 2. Light from the slit 2 passes through a plane mirror 7, a spherical mirror 10, a plane grating 5 and a spherical mirror 11 and emerges from a third slit 3. Since the dispersion of light takes place twice in two series-connected monochrometers including the gratings 4 and 5 respectively, that is, a monochromatic light at a certain wavelength emerging from the first monochrometer but containing stray light is again extracted from the second monochrometer as a monochromatic light of that wavelength, the stray light can be effectively eliminated. However, the double monochrometer shown in FIG. 1 has two drawbacks that many mirrors are used resulting in a very large total loss of light associated with reflection and that the degree of freedom for selection of the grating constants or angular dispersions of the plane gratings 4 and 5 is small. More especially, in order to minimize the total loss of light at slits included, the two monochrometers must be arranged in a reverse or subtractive dispersion relation with each other and moreover a considerable difference must be given between the degrees of dispersion (or angular dispersions) of the respective monochrometers, which requires different lengths of respective sine bars for driving the plane gratings 4 and 5. It is difficult to move such sine bars by means of a single feed screw mechanism in the optical system shown in FIG. 1.
Another conventional double monochrometer is shown in FIG. 2. A prism 12 is used as the first dispersion element in place of the plane grating 4 shown in FIG. 1. Since no components corresponding to the plane mirrors 6 and 7 of FIG. 1 are required, the number of reflecting planes used therein is reduced as compared with the optical system of FIG. 1. But, this double monochrometer has more or less drawbacks similar to those mentioned in conjunction with FIG. 1. Moreover, the use of the prism involves inherent drawbacks that the dispersion by a prism depends upon wavelengths or is smaller at longer wavelengths and that hence a sine bar cannot be used for driving the prism.
In still another example, the plane gratings 4 and 5 in FIG. 1 are replaced by prisms, respectively. This example involves the problem of reflection loss as mentioned in conjunction with FIG. 1 and the problem associated with the use of prism as mentioned in conjunction with FIG. 2.